Is PVU Needed?

[Avio]

Is PVU updating needed? I understand with EGI, PVU is not needed.

There was an occasion when I CUE the TPOD to look at where the radar was pointing (a small building), but it pointed a distance to the left of the building instead. Is there an inherent inaccuracy, or perhaps PVU is needed?

 

[SloppyDog]

2 hours ago, Avio said:

Is PVU updating needed? I understand with EGI, PVU is not needed.

There was an occasion when I CUE the TPOD to look at where the radar was pointing (a small building), but it pointed a distance to the left of the building instead. Is there an inherent inaccuracy, or perhaps PVU is needed?

 

Well, being Open Beta and still filled with bugs, I would say that's a bug. There shouldn't be any inaccuracy while using EGI. A problem that may have occurred is that the resolution of the radar screen is not enough to allow for a perfect or near perfect designation of the point where you want the TPOD to be cued at. 

Also, PVU do not account for INS drift. They are, as far as I know, made for compensate for errors in speed, which cause the radar images to look very blurry if you don't update the speeds.

What you should be doing to account for position errors would be a PVU and an INS update, using update points. But these only work, and are needed, when EGI is not available. Maybe you have hit the Present Position Keeping Source option and turned off the EGI or are in a mission where GPS is not available.

In summary:

1) Bug; or

2) Not enough radar resolution;

3) PPKS not EGI;

4) Mission where EGI is not available.

 

 

[Avio]

I ran the mission starting hot from runway and all default cockpit settings. I could be that the radar image was captured from too far a distance and full resolution has not been built yet.

 

[KlarSnow]

The radar even with EGI can still have velocity errors. An INS PVU can help if you are consistently getting blurry or messy maps. 
 

and just keep in mind the radar cannot determine elevation so it just pulls from the closest steerpoint or current steerpoint, so if you are just mapping random areas the radar will probly cue well above or below elevation wise whatever you designate.

[Hatman335]

On 8/2/2023 at 6:27 AM, SloppyDog said:

Also, PVU do not account for INS drift. They are, as far as I know, made for compensate for errors in speed, which cause the radar images to look very blurry if you don't update the speeds.

 

PVUs most certainly help with drift. Assuming you're running MN, it will get velocities from the INS and from a known starting position it integrates the velocities over time to calculate position changes and since it has a known starting position, adding those changes together will result in the system knowing your present position. If the velocities are inaccurate the system will essentially think that you're moving across the ground at a different rate than you actually are. Since this is provided by the INS using accelerometers the actual value can be slightly off and even slight errors can have huge impacts. (I'll give an example later)

 

The radar in turn directly measures this based on the rate of change of the ground returns and this is significantly more accurate. It then calculates delta values (the difference between the velocities it measured vs the velocities supplied by the INS) and adds these corrections to the MN for 5 minutes. After 5 minutes these values may no longer be valid so the system does not use them anymore. An MN PVU can give you sub knot accurate velocities. If all you're doing is just updating your position but keep the velocities inaccurate it will drift again anyway and if that happens the aircraft's understanding of the relative position of the target can also drift away rendering the designation inaccurate and hurting bombing accuracy as well.

 

SAR mapping also relies on knowing exact velocities since if there's a velocity error the Doppler shift is calculated incorrectly and the result will be a significantly degraded map. Therefore, having accurate velocities without EGI is crucially important.

 

INS PVU is another ballgame, that is typically done once per flight and it's purpose is to calibrate the INS to what its drift values are and its a bit more convoluted when performed properly than an MN PVU. This can lead to lowered drift over the course of the flight and it can also clear up the SAR image and this latter part is true even with EGI. Obviously if the mission has GPS available, drift will not be a concern and position updates are not required.

 

To sum it up, assuming all gets added properly and working as it should, do your INS PVU once per flight during your way to the target area or on the way to the tanker or whatever, use MN PPKS and do a PVU for every map and update and try to keep the time between the last PVU and the weapon release within 5 minutes if possible for the greatest accuracy. If running EGI doing an INS PVU once per flight should still help with the SAR image to some degree.

 

As an illustration, one knot of velocity error in the system is 1.7 feet of drift each second. So if you mapped the target at 10 nm and flew the attack run at 500 KTAS, by the time you get there the target would have drifted 120 feet. This gets further complicated if you add in squint angle error to the equation, but just to avoid making this message even longer, let's just agree to have the WSOman do those PVUs. Or just run EGI and let the geepus magically carry you to victory.

Edited August 3, 2023 by Hatman335

[SloppyDog]

Ok, thanks for the info. So even if you are running under EGI, a PVU from time to time won't hurt, since it is added to the INS calculations. Got it!

Also, do you have the sources to all this technical stuff? I don't want to rebuff what you said, just want to learn more about it.

 

[Hatman335]

31 minutes ago, SloppyDog said:

Ok, thanks for the info. So even if you are running under EGI, a PVU from time to time won't hurt, since it is added to the INS calculations. Got it!

 

INS PVU. I've learned most of this from Klarsnow and Notso on the Discord and the workbooks. I'm not an SME. The AGR series and the AGD-4 designations go into a lot of detail with this.

[Avio]

Thanks a lot, Hatman, for the very illuminating advice.

Pardon my ignorance, but how does one switch between MN and EGI as default? Or is EGI already the default? On the radar screen, it shows as MN at the top left screen.

And for INS PVU updating, does one really need to wait 3 to 6 minutes for a good accuracy update? Thank you.

 

[KlarSnow]

I dont think MN is implemented currently as a PPKS in the current DCS F-15E. Hopefully we get it implemented at some point because without EGI MN should be your primary PPKS, not INS. INS as a primary PPKS is a rare thing in the F-15E.

MN on the top right of the A/G Radar display should only show up if you are in update cursor mode. Then you can toggle that to update either the MN or the INS with the position update. Normally you would have it in MN and just update the MN if you were flying INS only.

When you select PVU it defaults to an MN PVU which can actually have benefits in DCS right now even though the MN is not a functioning separate PPKS at the moment. It can result in clearer/cleaner/easier to read maps, but you want to do the MN PVU shortly before making your map. The effect it has on cleaning up the picture only lasts a few minutes.

INS PVU no you don't need to wait for 3-6 minutes, as soon as you see values in the 0.1-0.2 region you can accept it and you will be good to go. In the real thing continuing a longer INS PVU for several minutes does have long term benefits so it was something performed every now and then. A quick INS PVU should be done every flight to help keep the radar and INS calibrated, a long one is performed after significant system changes (big radar maintenance, replacing the Central Computer or other significant computers or processing centers in the jet. replacing the INS etc...) because that will help calibrate the system over the course of its life by giving it a much more accurate correction for the pointing errors. Assuming that has already been done, a short one just to get the system calibrated and your values down to 0.2-0.1 or so is plenty good.

[Avio]

Thanks Klarsnow. Just to clear up some confusion, so it is recommended that in the current DCS F15E, it is advisable to do INS PVU updates especially each time before making a map?

I assume the model now is default to EGI. Should I wish to change to INS, how can that be done? Thanks.

 

[KlarSnow]

INS PVU once a flight. Especially if you are INS only. If you have EGI its not really necessary right now from what I can tell. 

IF you are INS only it is also probably advisable for best maps to do an MN PVU prior to every map. or at least one every few minutes that you are intending to do maps. the real thing dumps the correction values from the MN PVU after 5 minutes because it assumes at that point they are no longer valid, so you plan your MN PVU's and mapping legs so that you are doing a MN PVU within 5 minutes of every desired mapping point.

if you are EGI only unless you are having issues with your maps being unreadable you really shouldnt "need" to do either of these.

[Avio]

Ok thanks Klarsnow.

[Rainmaker]

13 hours ago, Hatman335 said:

PVUs most certainly help with drift. Assuming you're running MN, it will get velocities from the INS and from a known starting position it integrates the velocities over time to calculate position changes and since it has a known starting position, adding those changes together will result in the system knowing your present position. If the velocities are inaccurate the system will essentially think that you're moving across the ground at a different rate than you actually are. Since this is provided by the INS using accelerometers the actual value can be slightly off and even slight errors can have huge impacts. (I'll give an example later)

 

The radar in turn directly measures this based on the rate of change of the ground returns and this is significantly more accurate. It then calculates delta values (the difference between the velocities it measured vs the velocities supplied by the INS) and adds these corrections to the MN for 5 minutes. After 5 minutes these values may no longer be valid so the system does not use them anymore. An MN PVU can give you sub knot accurate velocities. If all you're doing is just updating your position but keep the velocities inaccurate it will drift again anyway and if that happens the aircraft's understanding of the relative position of the target can also drift away rendering the designation inaccurate and hurting bombing accuracy as well.

 

SAR mapping also relies on knowing exact velocities since if there's a velocity error the Doppler shift is calculated incorrectly and the result will be a significantly degraded map. Therefore, having accurate velocities without EGI is crucially important.

 

INS PVU is another ballgame, that is typically done once per flight and it's purpose is to calibrate the INS to what its drift values are and its a bit more convoluted when performed properly than an MN PVU. This can lead to lowered drift over the course of the flight and it can also clear up the SAR image and this latter part is true even with EGI. Obviously with EGI the INS doesn't drift so PP inaccuracies or drifting designations will not occur.

 

To sum it up, assuming all gets added properly and working as it should, do your INS PVU once per flight during your way to the target area or on the way to the tanker or whatever, use MN PPKS and do a PVU for every map and update and try to keep the time between the last PVU and the weapon release within 5 minutes if possible for the greatest accuracy. If running EGI doing an INS PVU once per flight should still help with the SAR image to some degree.

 

As an illustration, one knot of velocity error in the system is 1.7 feet of drift each second. So if you mapped the target at 10 nm and flew the attack run at 500 KTAS, by the time you get there the target would have drifted 120 feet. This gets further complicated if you add in squint angle error to the equation, but just to avoid making this message even longer, let's just agree to have the WSOman do those PVUs. Or just run EGI and let the geepus magically carry you to victory.

INS is still just INS. Dont confuse that with EGI that has its own internal INU. INS can still drift. 

[Hatman335]

8 hours ago, Rainmaker said:

INS is still just INS. Dont confuse that with EGI that has its own internal INU. INS can still drift. 

Sorry, what part is this referring to in particular? I may have misphrased something.

[Rainmaker]

8 minutes ago, Hatman335 said:

Sorry, what part is this referring to in particular? I may have misphrased something.

 

‘Obviously with EGI the INS doesn't drift so PP inaccuracies or drifting designations will not occur’

[Hatman335]

13 minutes ago, Rainmaker said:

‘Obviously with EGI the INS doesn't drift so PP inaccuracies or drifting designations will not occur’

Thanks, that's indeed pretty poor phrasing on my part, I'll fix that.  Have you found any other issues or inaccuracies that I should fix as well?